In recent years, in order to obtain an electron beam having higher brightness and longer operating life than a thermionic cathode, an electron source (hereinafter referred to as a ZrO/W electron source) employing a cathode made of a needle electrode of tungsten single crystal provided with a covering layer comprising zirconium and oxygen, has been used (Non-Patent Document 1).
The ZrO/W electron source is one wherein a diffusing source made of zirconium oxide is provided on a needle cathode of tungsten single crystal having an axial orientation being <100> orientation, so that zirconium and oxygen are diffused to form a covering layer (hereinafter referred to as a ZrO covering layer) so as to reduce the work function of the (100) crystallographic plane of tungsten single crystal to a level of from 4.5 eV to 2.8 eV. In this ZrO/W electron source, only the very small crystallographic facet corresponding to the (100) crystallographic plane formed at the forward end of the cathode becomes an electron emission area, whereby an electron beam having a higher brightness than by a conventional thermionic cathode can be obtained, and yet this electron source has such a characteristic that it has a longer operating life. Further, such an electron source has characteristics such that it is more stable than a cold field emission electron source and is operable even under a low vacuum degree and thus easy to use (Non-Patent Document 2).
As shown in FIG. 1, in the ZrO/W electron source, a needle cathode 1 of tungsten having <100> orientation which emits an electron beam, is fixed by e.g. welding to a predetermined position of a tungsten filament 3 provided on conductive terminals 4 fixed to an insulator 5. A zirconium- and oxygen-diffusing source 2 is formed at a portion of the cathode 1. Although not shown in the drawings, the surface of the cathode 1 is covered with a ZrO covering layer.
The cathode 1 is Joule heated by the filament 3 and used usually at a temperature of about 1,800 K. Accordingly, the ZrO covering layer on the surface of the cathode 1 will be lost by evaporation. However, from the diffusing source 2, zirconium and oxygen will diffuse and will be continuously supplied to the surface of the cathode 1, and consequently, the ZrO covering layer will be maintained.
When the ZrO/W electron source is used, the forward end of the cathode 1 is disposed between a suppressor electrode and an extractor electrode. To the cathode 1, a high voltage negative against the extractor electrode is applied, and to the suppressor electrode, a negative voltage at a level of a few hundred volts against the cathode 1 is applied to suppress thermionic electrons from the filament 3.
In a CD-SEM or a wafer inspection apparatus to be used at a low accelerating voltage, the ZrO/W electron source is operated at an angular intensity of from 0.1 to 0.2 mA/sr and shows the probe current and suppressed spread of energy width. For this reason, the ZrO/W electron source is widely used as an electron source enabling to obtain high-resolution SEM images.    Non-Patent Document 1: D. Tuggle, J. Vac. Sci. Technol. 16, p 1699 (1979)    Non-Patent Document 2: M. J. Fransen, “On the Electron-Optical Properties of the ZrO/W Schottky Electron Emitter”, ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. III, p 91-166, 1999 by Academic Press.